Graphene is a two-dimensional nanomaterial composed of pure carbon that forms a sheet consisting of a single layer of carbon atoms in a sp2 network arranged in a regular hexagonal pattern. The thickness of a graphene sheet is on the order of a single atomic unit, while its lateral dimension can approach up to tens of microns. Graphene has many desirable properties, including high electrical conductivity, good tensile and compressive strengths, excellent thermal properties, and it is light weight (at 0.77 milligrams per square meter). It is made of a lattice of 0.142 Nm-long carbon bonds. Such properties as strength and lightness make graphene a useful coating material for many applications.
A key aspect in the application of graphene is not only the production of the graphene, but the subsequent adhesion of the resultant graphene onto a surface of a substrate material. It is complex and costly for a substrate that is moving along a manufacturing production line to be coated with a continuous surface coating on a large commercial scale.
There are many ways to produce graphene. Most known methods are expensive and involve harsh chemicals and extreme environments. Initially, graphene was prepared at the individual sheet level by mechanical exfoliation from highly ordered pyrolytic graphite (HOPG) crystals, a method coined the Scotch tape method. The Scotch tape method is time consuming and produces only a small amount of graphene. Other synthetic methods attempt to improve on the Scotch tape method with the intent to scale up the production. For larger substrates, thin coats of high quality graphene have been prepared by epitaxial growth on SiC surfaces, and by chemical vapour deposition (CVD) on catalytic metal surfaces. For coating other types of substrates, costly and complicated transfer procedures are subsequently employed. Recently, graphene has been produced through chemical exfoliation of derivatized expanded graphite such as: graphite oxide; and halogenated graphene.
The use of graphene oxide (“GO”) is a viable and cost effective alternative in the production of graphene. GO is a promising precursor for bulk production of graphene-based materials because it can be synthesized in large quantities from inexpensive graphite powders. Graphite is reacted with strong oxidants to form graphite oxide, followed by gentle exfoliation or delamination to form GO. The resulting GO is light brown in color and is water dispersible and insulating. Graphene oxide sheets are being explored as a possible intermediate for the manufacture of graphene. GO can then be reduced to form a mixture of reduced graphene oxide (“r-GO”) and graphene. In such a mixture, a large portion of oxygen-containing functional groups have been removed by reactions with chemical reducing agents.
A challenge with deoxygenating treatments of GO in liquid phase is that, upon removal of the oxygen, the r-GO material is hydrophobic and therefore not dispersible in water and/or polar solvents. This hydrophobicity presents a challenge for coating processes based on the use of dispersions of r-GO. Thus, there exists a need for coating processes that produce graphene-like sheet coatings that are cohesive and adhere well to the surface being coated. Additionally, there exists a need for a process to coat materials with large surface areas with graphene-like coatings in a cost effective and environmentally benign manner.